Oxidation of substrates used in semiconductive elements has in the past been performed utilizing a variety of techniques to create insulating layers. Thin silicon dioxide films so formed have a wide range of application in the semiconductor industry. They have been formed by thermal oxidation of silicon, plasma enhanced chemical vapor deposition, known as glow discharge deposition, plasma anodization and plasma oxidation. Each of these processes has various advantages and disadvantages. For example, when thin silicon dioxide is formed by techniques of thermal oxidation higher quality films are obtained, especially the film properties at the silicon-oxide interface. However, high temperatures are required, typically in excess of 900.degree. C. which are too high for contemporary applications.
As the technology advances beyond VLSI and into ULSI devices of ultrahigh density, such as 64M bit and 256M bit DRAMS, low temperature processing is required to minimize unwanted dopant diffusion and lower film stress.
Thus, within semiconductor processing technology a variety of techniques have been proposed to achieve oxidation at temperatures below 700.degree. C. and in some cases even below 300.degree. C.
Reference is made to U.S. Pat. No. 4,323,589 which is directed to a process of growing oxide film on a silicon substrate utilizing plasma oxidation. In the '589 patent the process employs oxygen at a flow rate of 0.7-10 sccmm utilizing a relatively high r.f. power (1000-7000 W). It is well known that the use of high RF power, generally in excess of 200W, results in thicker oxide films. However, the films are generally of poor quality for ULSI applications. The '589 patent employs oxidation over a processing time ranging from 15 minutes to 8 hours with a high oxide growth rate achieving thicknesses of approximately 2600A. In accordance with the process in the '589 patent, oxidation occurs on the side of the wafer facing away from the plasma.
Reference is made to U.S. Pat. No. 4,576,829 also directed at a low temperature oxidation process. In the '829 patent plasma oxidation is carried out in a magnetically enhanced glow discharge system. The temperature of the silicon substrate is maintained at a temperature below 300.degree. C. during oxidation.
Techniques to enhance oxidation by the use of ozone are also established for a variety of different applications. It is, for example, well established that the use of ozone as a cleaning technique to expedite the removal of carbon deposits on surfaces allows the process to be carried out at relatively low temperatures. U.S. Pat. No. 2,443,373 is representative of a myriad of art which is directed at the use of ozone for purposes of cleaning surfaces. The cleaning of semiconductor wafers utilizing hydrochloric acid bubbled with vapor containing ozone is disclosed in JP58-100433.
Given the established qualities of ozone as a constituent for use in wafer cleaning, it has further been proposed to use an ozone enriched atmosphere for purposes of enhancing the formation of films on semiconductor devices. 68JP-092936 discloses the formation of an insulating film on a semiconductor by growing silicon dioxide utilizing the reaction of gaseous phase monosilane and ozone. 1973 JP-008434 describes the deposition of silicon dioxide films on a substrate by admitting ozone in a decomposable silicon CVD gas source into a reactor containing the substrate. The substrate is then irradiated using UV to allow low temperature operation.
While UV radiation will enhance the oxidation rate of a silicon substrate in an ozone environment, there are disadvantages associated with this technique. For example, at a medium UV light intensity the rate enhancement would be significantly lower than those of plasma enhancement due to much lower steady state reactive oxidation species in the oxidation ambient. As a result, the oxidation rate will be much lower. As the UV light intensity increases, the substrate will tend to heat up significantly thus losing the advantages of low temperature processing.
Moreover, high UV light radiation causes significant trapped electron/hole pairs in the silicon dioxide layer. This makes it unsuitable for not only ULSI but also VLSI fabrication without a subsequent high temperature annealing step. A more general problem with the use of UV radiation in the formation of semiconductor devices is that it is difficult to achieve a coherent and uniform UV light source beam in the range of 5-8 inches in diameter. Consequently, the oxidized layer thickness uniformity tends to be poor when compared with those produced with plasma oxidation.
The chemical deposition of an oxide film at very low temperatures with ozone in combination with monosilane is disclosed in 82JP-186956. In this technique oxygen is passed through an ionizer prior to passage into the reaction chamber. The technique allows the formation of an oxide film on a surface of a substrate which is decomposed at about 200.degree. C., for example, a synthetic resin.
East German Patent 81,916 discloses a process of oxidizing silicon surfaces by employing a mixture of oxygen and ozone. However, the ozone generation takes place in the r.f. reactor tube itself through an r.f. electrical discharge. This process is basically identical to those of U.S. Pat. No. 4,323,589 where ozone is also generated in the oxygen r.f. glow discharge. As a result high r.f. power, far exceeding 200W, is needed to sustain sufficient reactive ozone in the discharge for any oxidation below 400.degree. C. Consequently, the oxide film grown by this kind of discharge will have inferior quality and be unsuitable for ULSI fabrication. In the '916 disclosure, the silicon surface is heated to a temperature of approximately 100.degree. C. As in other techniques, the oxidizing effect of ozone is also enhanced by U.V. radiation.
Another example of forming insulating films utilizing oxygen and ozone is disclosed in U.K. patent specification 1,274,699. In accordance with the '699 disclosure, a silicon dioxide film is grown by gaseous phase reaction of monosilane and ozone with a reaction temperature in the range of 150.degree.-300.degree. C.
The use of plasma deposition for semiconductor processing has been proposed but has achieved limited commercial success. This is because of the difficulty in controlling plasma processing parameters and the complexity of the phenomena involved. For early steps of ULSI fabrication, the film's quality has been considered too low to compete with other methods. Similarly, plasma oxidation processing has also remained an object of study but has not achieved commercial utility.
The primary role of plasma in the production of semiconductor devices is to produce chemically active specie that subsequently are deposited on or oxidize the surface under processing. In general, the plasma used for semiconductor applications is produced by the application of a high frequency electric field across a body of gas. When the plasma process commences energy from the electric field is coupled into the gas almost entirely via the kinetic energy of a few free electrons. The electrons acquire energy from the field and lose it rather slowly as a consequence of elastic collisions. Those electrons become capable of inelastic collision and ionize, or dissociate gas molecules to produce secondary electrons by electron impact reactions. A key factor is that the substitution of electron kinetic energy for thermal energy avoids excessive heating and consequent degradation of substrates. Thus, plasma processing is attractive in that it can be operated at lower temperatures. Recent reviews of various known plasma techniques for the oxidation and deposition of materials on silicon is presented in "Plasma Assisted Chemical Vapor Deposited Thin Films for Microelectronic Applications", S. V. Nguyen, J. Vac. Sci. Technol. B. 4.(5), sep/oct 1986, 1159-1167 and "Handbook of Thin-Film Deposition Processes and Techniques" edited by Klaus K. Schuegraf, Noyes publication pp. 112-146 and pp. 395-408, published in 1988. Reference is also made to other reports in the literature directed at the use of plasma in the deposition of silicon oxide films by the use of a remote plasma enhanced chemical vapor deposition (RPECVD), PPD Richard, et al, "Remote Plasma Enhanced CVD Deposition of Silicon Nitride and Oxide for Gate Insulators in (Rn,Va) As FET devices", J. Vac. Sci. Technol. A.(3), May/Jun 1985, 867-872; Lucovsky, et al "Deposition of Silicon Dioxide and Silicon Nitride by Remote Plasma Enhanced Chemical Vapor Deposition", J. Vac. Sci. Technol. A 4(3), May/June 1986, pp. 681-688; and Tsu, et al, "Silicon Nitride and Silicon Dimide Grown by Remote Plasma Enhanced Chemical Vapor Deposition", J. Vac. Sci. Technol. A 4(3), May/Jun 1986, pp. 480-485.
The ability to reduce processing temperatures in the creation of oxides is important since it minimizes dopant diffusion and also reduces stress in the film. These two factors potentially allow the formation of thinner oxides. Although plasma deposition has been used to deposit oxide films suitable as interlevel metal dielectric for ULSI devices, however, to date both plasma oxidation and deposition, despite the theoretical advantages, have not been demonstrated to be practically effective for high quality thin gate oxide, node dielectric or sidewall spacer fabrication for ULSI devices.